1. Field of the Invention
The invention relates to laminates which contain at least one layer of thermoplastic polysiloxane-urea copolymers and at least one further layer of one or more solar cell units, a process for the production thereof and their use.
2. Description of the Related Art
In general, a multilayer material which forms as a result of lamination, i.e. pressing and simultaneous adhesive bonding, of at least two layers of identical or different materials is designated as a laminate (lat. lamina: layer). Glass laminates have long been known and are used widely as so-called laminated safety glasses in automotive, vehicle and aircraft construction and in the building industry. Laminated glass has a sandwich-like composition and consists of at least one glass panel and a polymeric layer present thereon. Plasticized polyvinyl butyral (PVB) in sheet form is most frequently used as the polymeric intermediate layer. Furthermore, the intermediate layer may also consist of polyurethane or polyacrylate or of a combination of a plurality of different materials.
Photovoltaic solar modules are laminates which have a layer-like structure similar to the laminated glasses but additionally contain at least one photosensitive semiconductor layer which is connected in a suitable manner via contacting tracks to one or more photovoltaic cells (referred to below as “solar cell units”). Laminates having this structure are also known in general usage by the terms “photovoltaic module”, “solar cell module”, “solar module”, “solar panel” or the like. In this context, reference may be made, for example, to Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1992, vol. A24, pages 393-395.
A photovoltaic solar module consists as a rule of one or more solar cell units which are connected to one another and are provided with a transparent protective covering for protection from external influences. Here, the solar cell units are frequently laminated between a glass plate and a more or less rigid, rear covering sheet which can likewise consist of glass or organic polymers/copolymers, such as, for example, those based on polyvinyl fluoride (PVF) or polyethylene terephthalate (PET), with the aid of a transparent adhesive layer (rigid solar module). In addition, flexible solar modules which are bendable within certain limits are also known. These consist of the front, protective cover layer, for example transparent organic (co)polymers, while the rear covering sheet consists of a thin metal or plastic sheet or a suitable plastic-based and/or metal-based composite material.
The transparent adhesive layer required for lamination has a plurality of functions and the requirements which the materials must meet are accordingly high. Thus, the adhesive layer acts firstly as a protective embedding material for the extremely sensitive solar cell units by completely enclosing them and substantially insulating them from external influences, such as the penetration of moisture and oxygen. At the same time, however, it must not adversely affect the optical properties of the photosensitive materials. Furthermore, the embedding material used must be highly transparent and UV-stable over decades and must guarantee the permanent cohesiveness of the sandwich-like material composite over the entire life time of the photovoltaic solar module. Further requirements are simple processibility, good adhesion to the relevant substrates and high transparency and freedom from bubbles after the lamination step.
Organic casting resins, for example based on polyurethanes, polyesters, polycarbonates, epoxides and acrylates, and crosslinkable silicone-based systems, such as, for example, silicone gels, are frequently used as transparent adhesive or embedding material. These adhesive systems can be adjusted in the uncured state to have such a low viscosity that even very small cavities are completely filled and the solar cell units enclosed without bubbles. Vulcanization is then induced by curing agents or crosslinking agents already present in the adhesive system or later introduced, and a mechanically strong adhesive layer is obtained.
A disadvantage of these embedding materials is the complicated production of corresponding solar modules, since the lamination step requires the handling of multicomponent systems and considerable care when placing or potting the solar cell units; this applies in particular to large-area elements. In the case of the organic casting resin systems, curing is moreover a process which is difficult to control. In addition, some of the casting resins have a tendency toward bubble formation, cloudiness or delamination after years.
An alternative to curing systems is—in analogy to the production of laminated safety glass—the use of thermoplastic films based on organic polymers or copolymers, in particular on polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA). For this purpose, the solar cell units are embedded between the polymer sheets and then bonded at elevated pressure and elevated temperature with the desired covering materials to give a laminate. The use of thermoplastic EVA or PVB sheets is, however, associated with some disadvantages which adversely affect both the quality and the production costs of photovoltaic solar modules.
The EVA widely used in solar module construction contains organic peroxides in order to subsequently crosslink the originally thermoplastic material during the lamination step and thus substantially to improve the creep resistance of the embedding material. However, the peroxide is often not completely consumed during the lamination, so that any excess peroxide can promote subsequent oxidation or decomposition, but in particular, yellowing of the EVA. Thus, for example, it is known that EVA subsequently crosslinked with peroxide becomes yellow under extensive exposure to sunlight over the years, as occurs, for example, during the operation of solar modules in the open air. However, this leads to a gradual decrease in the solar energy efficiency of the module. Furthermore, owing to the peroxide-induced postcrosslinking which takes place simultaneously, the lamination step must be affected in vacuo. The reason for this lies in the fact that atmospheric oxygen adversely affects the free radical crosslinking reactions and reduces the degree of crosslinking and hence the creep resistance of the postcrosslinked EVA. The organic peroxides in turn, like their cleavage and degradation products, are relatively reactive compounds which often lead to premature wear of the membranes present in the EVA lamination apparatuses. In addition, under production conditions, EVA may release small amounts of acetic acid which may subsequently promote metal contact corrosion in the solar cell units. In addition, the melting, the relatively slow crosslinking of the EVA and the lamination of the material composite at about 150° C. and in vacuo lead to cycle times of about 15 to 30 minutes per module.
The PVB widely used at the beginning of the development of solar module construction likewise has a serious disadvantage: PVB is known to be extremely hygroscopic and has a strong tendency toward moisture absorption.
It must therefore be stored under defined, climatic conditions up to the lamination step. Furthermore, it is known from adverse experience from the laminated glass sector that PVB can slowly absorb moisture even in the laminated state and thus becomes cloudy to a greater or lesser extent. The cloudiness in turn reduces the light transmittance of the laminate layer significantly so that, in such a case, the solar energy efficiency of the module is markedly reduced. In order therefore to keep the influence of atmospheric humidity on the laminating medium as small as possible for the complete life time of the solar module, special sealing systems or additional edge seals comprising other sealants are required. This makes the construction of corresponding solar modules complicated and expensive.
A thermoplastic polyurethane system (TPU) which can be processed just as easily as PVB but at the same time is said not to have the problems described above which are known for PVB has been described as an alternative to PVB. In this context, reference may be made to DE 20220444 U1. The thermoplastic polyurethanes used are reaction products of aliphatic diisocyanates with organic polyols, it being possible, if appropriate, for additional chain extenders based on organic polyols to be present. A disadvantage of the adhesive system described is, however, the use of purely organic synthesis building blocks, which results in reduced long-time UV stability and therefore necessitates the addition of additional UV stabilizers. This in turn—incidentally as in the case of EVA—leads to a reduced transparency of the laminates in the high-frequency visible and UV-A and UV-B range which is of interest in particular for solar cells not based on silicon. Furthermore, the use of organic polyols as a synthesis building block is disadvantageous as these compounds are known to be polar and hydrophilic, which counteracts a water vapour diffusion barrier effect and thus long-term weathering stability of the laminates disclosed. Furthermore, the class of substances used and comprising TPUs has a relatively high glass transition temperature and hence limited flexibility, in particular at low temperatures. However, since the modulus of elasticity close to the glass transition temperature is strongly temperature-dependent, the result of this may be that, during operation of such a solar module at low outside temperature (e.g. below 0° C.) the adhesive layer becomes very taut and the fragile solar cells and the connecting conductors are also adversely affected, which can lead to total failure of the module or incipient delamination.